Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butylenes, butadiene, and aromatics such as benzene, toluene, and xylenes. Each of these is a valuable commercial product in its own right. For instance, the olefins may be oligomerized (e.g., to form lubricant basestocks), polymerized (e.g., to form polyethylene, polypropylene, and other plastics), and/or functionalized (e.g., to form acids, alcohols, aldehydes and the like), all of which have well-known intermediate and/or end uses.
Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Typically, the feedstock for steam cracking is a hydrocarbon such as naphtha, gas oil, or other non-resid containing fractions of whole crude oil, which may be obtained, for instance, by distilling or otherwise fractionating whole crude oil. Conventional steam cracking utilizes a steam cracking furnace which has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock (and optional steam) mixture is then conveyed into the radiant section where the cracking takes place. Typically the vaporized mixture is introduced through crossover piping into the radiant section where it is quickly heated, at pressures typically ranging from about 10 to about 50 psig (69 to 345 kPa), to a severe hydrocarbon cracking temperature, such as in the range of from about 1450° F. (788° C.) to about 1650° F. (900° C.), to provide thorough thermal cracking of the feedstream. The resulting products, including olefins, leave the steam cracking furnace for further downstream processing.
After cracking, the effluent from the pyrolysis furnace contains a variety gaseous hydrocarbons, e.g., saturated, monounsaturated, and polyunsaturated, and can be aliphatic and/or aromatic, as well as significant amounts of molecular hydrogen. The cracked product is then further processed such as in the olefin production plant to produce, as products of the plant, the various separate individual streams of high purity, i.e., hydrogen, the light olefins ethylene, propylene, butylenes, and aromatic compounds, as well as other products such as pyrolysis gasoline.
As worldwide demand for light olefins increases and the availability of favorable crude sources is depleted, it becomes necessary to utilize heavier crudes (i.e., those having higher proportions of resid), which requires increased capital investments to process and handle the refining byproducts. It is highly desirable to have processes that can take lower cost, heavier crudes, and produce a more favorable product mix of light olefins, more efficiently. However, conventional steam cracking processes are known to be prone to severe fouling by feedstocks containing even small concentrations of resid, which is commonly present in low quality, heavy feeds. Thus, most steam cracking furnaces are limited to processing of higher quality feedstocks which have had substantially all of the resid fraction removed in other refinery processes. Such additional processes increase the cost of the overall process. Likewise, removal of the resid fraction lowers the overall conversion efficiency of the refinery process, since most of the resid fraction is mixed with low value fuel oils, rather than being converted to higher-value materials.
U.S. Patent Published Patent Application No. 2007/0090018, incorporated herein by reference, discloses integration of hydroprocessing and steam cracking. A feed comprising crude or resid-containing fraction thereof is severely hydrotreated and passed to a steam cracker to obtain an olefins product.
Cracking of heavy hydrocarbon feeds in fluidized cokers has been described. For example, U.S. Pat. No. 3,671,424, incorporated herein by reference, discloses a two-stage fluid coking process in which the first stage is a transfer line for short contact time and the second is either a transfer line or a fluidized bed.
The use of knock-out drums integrated with the steam cracking furnace has evolved as an important extension of this platform to enable processing of heavier feedstocks such as atmospheric resids. The knock-out drum provides a means to separate the heaviest components from crackable gas oil molecules and prevent the heaviest, asphaltene-type molecule containing fractions from fouling the steam cracking furnace. Unfortunately, by using this approach, most of the heavy vacuum resid molecules, which are favored as feedstock due to lower cost, remain in the liquid phase and are not converted in the radiant section of the steam cracking furnace.
Other patents of interest include U.S. Pat. Nos. 7,097,758; 7,138,047; 7,193,123; 3,487,006; 3,617,493; 4,257,871; 4,065,379; 4,180,453; 4,210,520; 3,898,299; 5,024,751; 5,413,702; 6,210,561; 7,220,887; US 2007/023845; WO 01/66672; WO 2007/117920; U.S. Pat. Nos. 6,632,351; 4,975,181; WO 2009/025640; US 2007/0090018 and WO 2007/117919. Other references of interest include: “Tutorial: Delayed Coking Fundamentals.” P. J. Ellis and C. A. Paul, paper 29a, Topical Conference on Refinery Processing, 1998 Great Lakes Carbon Corporation (which can be downloaded from http://www.coking.com/DECOKTUT.pdf).
There remains in the art a need for new means and improved processes for economical processing of heavy, resid-containing feeds for the production of olefins, aromatics, and other valuable petrochemical products. Likewise there remains a need in the art for means to upgrade resid to a more useful and/or efficient composition.
This invention discloses a method for producing chemicals from heavy feedstocks in a manner where significant portions of the vacuum resid are converted to lighter molecules which can be more easily vaporized in the knock-out drum and subsequently converted to fuels and chemicals.